Researches On Theoretical Model, Numerical Simulation And Application Of Painting Removal By Laser Cleaning

Laser cleaning is a new type of surface cleaning technique. Compared with traditional cleaning techniques, the technique has excellent cleaning effect and is environmental friendly, so it attracts more attention and has achieved great improvement since the first use of last century. According to whether assistant materials are used or not during cleaning process, laser cleaning can be divided into two different types:dry laser cleaning and steam laser cleaning. Dry laser cleaning has unique cleaning mechanisms and advantages that exceed traditional cleaning techniques. More and more scientists and engineers have been attracted into this domain, and theoretical models and cleaning devices have been developed and studied successively.In this dissertation, theoretical modeling of paint removal by dry laser cleaning was chosen as research object, to overcome the shortage of the existing cleaning models, especially the thermal expansion model with the shortcoming of simple structure, many assumed condition and the neglect of the effect of interlayer. Based on the existing removal model of large area coating a new thermal ablation vibration model is constructed. The model has three absorptive layers and is more accord with practical situations. Through this model, the mechanisms of laser paint stripping are further analyzed. Moreover other coatings removal threshold and key factors that influence the actual cleaning effect are simulated. In this model of three absorptive layers, the absorbencies of each layer are defined and the heat converting process is analyzed. With the computer aid of finite element method, the theoretical cleaning threshold, temperature, stress, and displacement distribution be calculated. A series of experiments of dry laser paint stripping also be carried out and measured. The experimental and theoretical results are compared at different working environments. The results demonstrated that this model is effective. The cleaning effect can be better and the thermal damage of substrate can be reduced if the interlayer is taken into account appropriately.Through beam shaping, flattened laser beam can be obtained from Gaussian beam. The surface temperature, the thermal stress, and the displacement of paint and substrate layer are numerically calculated, which are resulted from pulsed laser heating of Gaussian and flattened laser beam respectively. Comparing these theoretical results, we can find out differences between these two kinds of laser energy distribution. On the basis of former numerical calculation, the cleaning effects and efficiency of different working are compared. The results demonstrated that the high repetition flattened pulsed laser can reduce the substrate damage, and the cleaning efficiency can be increased about seven times.In this dissertation, the application of laser cleaning is also studied. The optimization method of dry laser cleaning is investigated by control the laser parameters, including laser fluence, beam shape, and angle of incidence. Through the change of intensity of laser and overlap ratio of circle laser spot, the optimized cleaning efficiency can be obtained, especially with the overlapped ratio of80%and more.In this dissertation, the model of three layer thermal ablation vibration is helpful to enhance the laser cleaning efficiency in view of interlayer. The investigation of flattened laser can improve the stripping rate and reduce the substrate damage. Moreover the studies of laser cleaning processes have great value to paint stripping and recoating in industrial protection.